1. Field of the Invention
The present invention is directed to a method and apparatus for producing light conductive fibers which have a core disposed in a loose fitting cladding tube.
2. Prior Art
A light conductor fiber, which has a core of highly refined quartz glass disposed in a loose fitting sleeve or tube of synthetic material is disclosed by P. Kaiser, A. C. Hart, Jr. and L. L. Blyler, Jr., "Low-Loss FEP-Clad Silica Fibers," Applied Optics, Vol. 14, No. 1, Jan. 1975, pp. 156-162. However, in these known light conductor fibers, a series of problems may occur. For example, contamination at the surface of the core can occur during the production of the loose fitting sleeve or tube on the light conducting cores. These contaminations will lead to an increased dispersion in the fiber so that the properties of the fibers are not reproducible. Moreover, the materials such as water can diffuse through the tube or sleeve of synthetic material and contaminate the core of the fiber. Furthermore, a danger, that the synthetic material of the tube will crystallize out and cause an increase of the value of the dampening of the light conductor fiber, also exists. Since the synthetic material of the sleeve or tube already has a relatively high dampening value, the light conducting fibers will possess only a low numerical aperture.
Another well known type of light conducting fiber comprises a core, which generally consists of a highly refined glass, and a tightly fitting sleeve or cladding which consists of a less refined glass. The compounds of the glass for the core is selected in such a manner that the refractive index of the core material is larger than the refractive index of the sleeve or cladding. Since the core and cladding form a compact unit and since it is possible during the stretching or drawing of such a light conductive fiber for the fiber core to be moved from an exactly concentric position within the cladding, considerable difficulties can arise during the connection of two such fibers such as during calibration. Any small alteration of the geometry of the fiber end surfaces, which are to be connected, for example one core being offset from the axis of the other core, will lead to a high coupling loss. Moreover, the space factor of the core is decreased by the presence of the cladding during a splicing of several individual fibers within a tight fitting sleeve. Each of these difficulties is discussed by M. K. Barnoski, "Data Distribution Using Fiber Optics," Applied Optics, Vol. 14, No. 11, November 1975, 11. 2571-2577.
In order to obtain an acceptance angle, which is as large as possible during coupling of light into a light conductive fiber, the difference between the index of refraction of the core relative to the cladding must be as large as possible. However, since the thermal and chemical properties of the different glasses for the cladding and for the core have to be precisely adjusted to one another, the difference in the index of refraction of the core relative to the cladding for a light conductive fiber with a tight fitting cladding cannot be generally increased in an arbitrary fashion. When the materials, which are used for the core and cladding, have too large a difference in their thermal coefficients of expansion, problems with cracks occurring during the formation of the light conducting fibers will be present. Chemical incompatabilities of the different glasses used for the core and cladding can lead to crystallite formations and phase separation at the boundary areas between the core and cladding.
In an attempt to overcome these problems, an article by T. Miyaskita, T. Edahiro, S. Takahashi, M. Horigushi and K. Masuno, entitled "Eccentric-Core Glass Optical Waveguides," Journal of Applied Physics, Vol. 45, No. 2, February 1974, pp. 808-809, suggested an eccentric-core optical fiber in which highly refined core of quartz glass was melted or attached at only one line to a boron silicate sleeve or tube. However, in this type of fiber construction, chemical and thermal properties of the core and the cladding tube or sleeve have to be adjusted to one another. In addition, the production of these fibers is accomplished with a rod-tube method, which uses a workpiece that comprises a glass rod for the core inserted into a glass tube for the sleeve or cladding and the rod-tube workpiece or unit is subsequently heated and drawn into a fiber. As a result of this method, reproducible results for the fibers can only be obtained with difficulties.